CBD-1207:  use v8 revision "3.21"

Change-Id: Idb57ed74265c53a85a8f9038308aee7406be2038
Reviewed-on: http://review.couchbase.org/33023
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>